1. Field of the Invention
This invention relates to a constant voltage generating circuit which converts the output voltage of an unregulated voltage source into a constant high voltage.
2. Description of the Prior Art
For example, the constant high voltage generating circuit for operating a photomultiplier or the like has included a plurality of series-connected control transistors and a circuit for detecting any error of the output of the constant or regulated voltage generating circuit, whereby the collector-emitter resistances of the control transistors have been controlled by the output of the detecting circuit to convert the voltage of an unregulated voltage source into a constant voltage. The purpose of the plurality of control transistors used in the above-described circuit lies in that if a high voltage is applied to the control transistors as the result of the output of the circuit being short-circuited, the transistors divide such high voltage so that they may be individually prevented from being broken down by the voltage. In the constant voltage generating circuit of the described type, however, the base currents of the plurality of control transistors are all supplied from an unregulated voltage source and this has led to a correspondingly great capacity required of the unregulated voltage source, and also to the disadvantage of a limited range within which the output voltage of the constant voltage generating circuit is variable. On the other hand, to overcome such disadvantages, there has been proposed a constant voltage generating circuit in which a photocoupler responsive to the output of the above-described error detecting circuit and a voltage source for supplying the base currents of the plurality of control transistors are provided separately so that the base currents may be controlled by the photocoupler, but the circuit of this type has these disadvantages: since the output of such constant voltage circuit is a high voltage, and accordingly, the separate voltage source is of a high voltage, the separate voltage source must be capable of withstanding a high voltage and thus must have a high dielectric breakdown voltage property with respect to the earth, and the same is true of the photocoupler. Thus, the conventional constant voltage circuit of the latter type has suffered from a disadvantage in that a high voltage withstanding property is required of the circuit for transmitting the output of the error detecting circuit to the inputs of the control transistors. Further, the use of the photocoupler has led to a limited range within which the inputs of the control transistors can be highly accurately controlled in accordance with the error detecting circuit, and accordingly has led to a limited range within which the output of the constant voltage circuit is controlled.